Fuel cells are used as a power source for electric vehicles, stationary power supplies and other applications. One known fuel cell is the PEM (i.e., Proton Exchange Membrane) fuel cell that includes a so-called MEA (“membrane-electrode-assembly”) comprising a thin, solid polymer membrane-electrolyte having an anode on one face and a cathode on the opposite face. The MEA is sandwiched between a pair of electrically conductive contact elements which serve as current collectors for the anode and cathode, which may contain appropriate channels and openings therein for distributing the fuel cell's gaseous reactants (i.e., H2 and O2/air) over the surfaces of the respective anode and cathode.
PEM fuel cells comprise a plurality of the MEAs stacked together in electrical series while being separated one from the next by an impermeable, electrically conductive contact element known as a bipolar plate or current collector. In some types of fuel cells each bipolar plate is comprised of two separate plates that are attached together with a fluid passageway therebetween through which a coolant fluid flows to remove heat from both sides of the MEAs. In other types of fuel cells the bipolar plates include both single plates and attached together plates which are arranged in a repeating pattern with at least one surface of each MEA being cooled by a coolant fluid flowing through the two plate bipolar plates.
The fuel cells are operated in a manner that maintains the MEAs in a humidified state. The level of humidity or hydration of the MEAs affects the performance of the fuel cell. Too wet of an MEA limits the performance of the fuel cell stack. Specifically, formation of liquid water impedes the diffusion of gas to the MEAs, thereby limiting their performance. The liquid water also acts as a flow blockage reducing cell flow and causing even higher fuel cell relative humidity which can lead to unstable fuel cell performance. Additionally, the formation of liquid water within the cell may cause significant damage when the fuel cell is shut down and exposed to freezing conditions. That is, when the fuel cell is nonoperational and the temperature in the fuel cell drops below freezing, the liquid water therein will freeze and expand, potentially damaging the fuel cell. Too dry of an MEA also limits the performance. Specifically, as the humidity level decreases the protonic resistance of the MEA will start to increase (especially near the inlet), resulting in additional waste heat and lower production of electricity. Furthermore, durability data suggests that large cycling in the moisture content of the MEA that leads to repeated flooding and drying of membranes can lead to significant loss in durability due to membrane swelling and shrinking. Thus, repeated flooded and dry operating conditions lead to a loss of overall efficiency and may reduce the durability of the MEA and the fuel cell.
Accordingly, it is advantageous to control the operation of the fuel cell in a manner that allows for efficient operation of the fuel cell and/or minimizes an impact on the durability of the MEA and fuel cell. Prior control strategies to manage the operation of the fuel cell have focused on maintaining a cathode effluent relative humidity at a constant level. Such strategies, however, do not monitor the state of hydration of the fuel cell and/or fuel cell stack (i.e., how much water buffer is in the membrane, diffusion media and channels). Additionally, the prior control strategies do not actively manage process excursions that may lead to drying and flooding of the fuel cell and/or fuel cell stack.